This invention concerns two-cycle engines, and more particularly loop scavenging type two-cycle engines with improvements made to scavenging passages.
This type of loop scavenging two cycle engine 51 known in the prior art has the construction shown in FIG. 12 wherein scavenging passages 56 extend upward from the top of a crank chamber 55 inside the sidewall of a cylinder 53, the passages 56 connect with scavenging charge ports 57 inside the cylinder 53, and the piston 54 is lowered to charge the fuel mixture from the inlet port into the crank chamber 55 and to the cylinder 53 from the scavenging charge ports 57 via the scavenging passages 56. Such a construction is disclosed, for instance, in Japanese Patent Publication Sho60-48609.
Another construction of a two-cycle engine is disclosed in Japanese Utility Model Kokai Sho57-13217 wherein a passage from the bottom of the crank chamber to the top surface of the crankcase is formed through the crankcase sidewall, and the passage is connected to the scavenging passage on the cylinder side to supply the fuel mixture to the crank chamber to the cylinder.
Of the engines discussed above, the engine shown in FIG. 12 has the inner diameter (the cross sectional area in the horizontal direction) of the scavenging passage 56 formed substantially vertical inside the sidewall of the cylinder 53 from above the crank chamber which is substantially the same as that of the area of the opening of the scavenger port 57. Therefore, as the fuel mixture forced outside the crank chamber 55 rapidly flows into the cylinder 53 from the scavenging charge port 57, a portion of unburned fuel mixture is emitted to the atmosphere from the outlet port 59 with the exhaust gas, the emitted amount being more than 30% of the scavenging gas flowing into the cylinder 53. This is criticized as hazardous to the natural environment.
The engine disclosed in Japanese Utility Model Kokai No. Sho57-13217 has a drawback in that the smooth supply of scavenging gas is prevented or the supply to the cylinder is altogether suspended by the negative pressure inside the scavenging passages as the rotational speed of the engine increases and the scavenging passages resonate.